High current density nitrogen implantation of an austenitic stainless steel

Surface modification of AISI 304L austenitic stainless steel under high flux and low energy nitrogen ion implantation has been carried out at moderate temperatures in the range 270–400 °C with 1.2 keV ions and current densities between 0.5 and 1 mA/cm2. The influence of temperature, current density and fluence on the nitrogen transport and the microstructure of the nitrided layer have been investigated. The nitrogen depth profiles have been determined by nuclear reaction analysis, the microstructure of the modified layers has been analysed by X-ray diffraction and transmission electron microscopy. It is shown that the processing temperature and the ion flux have a major influence on both the profile shapes and the unusually deep penetration depth of nitrogen. The results suggest that above a critical temperature the nitrogen transport increases rapidly giving rise to a flat depth profile. The formation of the well known expanded austenite γN has been observed and its structure identified to a f.c.c. lattice containing a high density of stacking faults induced by the high level of internal stresses. The X-ray photoelectron spectroscopy study of the Cr2p3/2, Fe2p3/2 and N1s binding states demonstrates clearly the preferential bonding of chromium with nitrogen. The atomic transport of nitrogen and the profile shapes are discussed in relation with both the specific role of Cr and additional processes such as the formation of surface vacancies and adatoms.